Comprehensive Guide to Fluid, Electrolyte, and Acid-Base Balance

Principles of Fluid and Electrolytes

Electrolytes are vital substances found in urine, blood, tissues, food, drink, and supplements. They are fundamentally responsible for maintaining the homeostasis of water and $pH$ levels, moving waste out of the body, and moving nutrients into body cells. Furthermore, they facilitate the proper function of the nerves, the heart muscle, general body muscles, and brain function. Fluid within the body serves several critical roles, including carrying nutrients and waste products, regulating and maintaining temperature, and acting as a lubricant, insulator, and shock absorber.

Total body water is significantly affected by age, sex, and individual size. A higher percentage of body fat results in a lower percentage of body fluid. Body fluid is divided into two primary compartments. Intracellular fluid refers to fluid inside the cells, accounting for approximately $40\%$ of total body weight and two-thirds of the total body fluid in adults. The primary electrolytes found in this compartment are potassium and phosphate. Extracellular fluid refers to fluid outside the cells and is subdivided into intravascular, interstitial, and transcellular compartments. It represents one-third of total body fluid, with primary electrolytes being sodium, calcium, and magnesium.

Fluid Balance and Thirst Stimuli

Water balance is the equilibrium between the intake and excretion of fluids. A body is in water balance when the input and output of water are equal. Approximate values for water intake include $1,200\,ml/day$ from fluids, $1,000\,ml/day$ from foods, and $300\,ml/day$ produced metabolically, totaling $2,500\,ml/day$. Output typically equals this intake, with $700\,ml/day$ lost via insensible means, $100\,ml/day$ from sweat, $200\,ml/day$ from feces, and $1,500\,ml/day$ through urine.

Thirst is defined as the physiological urge to drink water. There are four major stimuli to thirst. Hypertonicity involves cellular dehydration acting through an osmoreceptor mechanism in the hypothalamus. Hypovolaemia occurs when low volume is sensed by low-pressure baroreceptors in the great veins and right atrium. Hypotension is detected by high-pressure baroreceptors in the carotid sinus and aorta. Finally, Angiotensin II is produced as a result of renin release by the kidney, often in response to renal hypotension.

Kidney Function and Hormonal Regulation of Body Fluids

The kidneys perform several essential functions to maintain balance: they clear waste, regulate blood pressure for fluid balance, concentrate urine, regulate red blood cell (RBC) production, and regulate acid-base levels. Hormonal regulation involves various systems. Antidiuretic hormone (ADH), also known as vasopressin, is secreted by the pituitary gland when the brain's thirst center is stimulated; it prompts the kidneys to produce less urine. The Renin-Angiotensin-Aldosterone mechanism is triggered by low fluid or sodium ($Na^+$) levels, where renin from the kidneys and angiotensinogen from the liver lead to the production of Angiotensin II. This causes widespread arterial vasoconstriction and stimulates the secretion of ADH and aldosterone, increasing water reabsorption.

Natriuretic peptides are produced and stored in the heart and released in response to pressure changes and fluid volume in the heart. Atrial Natriuretic Peptide (ANP) acts as a defense against excessive water by inhibiting aldosterone, serving as a potent diuretic, inhibiting thirst, and promoting sodium wasting. Brain Natriuretic Peptide (BNP) measures heart pressure and can indicate heart failure if elevated.

Movement of Body Fluids and Tonicity

Fluid movement involves solvents (the dissolving medium) and solutes (the substance being dissolved). Osmosis is the movement of fluid from an area of low solute concentration to an area of high solute concentration; where $Na^+$ goes, water follows. Osmotic pressure refers to the power of a solution to draw fluid across a membrane, while Colloid osmotic pressure (Oncotic pressure) involves plasma proteins like albumin pulling fluid from the interstitial space into the vascular space. Osmolarity is the total milliosmoles per liter of solution (volume), while osmolality is the number of milliosmoles per kilogram of solution (weight), which is the preferred measure.

Fluid deficit is generally characterized by an osmolality exceeding $295\,mOsm/kg$, while fluid excess is indicated by an osmolality below $275\,mOsm/kg$. Tonicity refers to the osmolality of a solution. Isotonic solutions have the same osmolality as blood, so fluid stays in the intravascular compartment (examples include Normal Saline or NS, Lactated Ringer's or LR, and $D_5W$). Hypotonic solutions have lower osmolality than blood, moving fluid into cells (examples include $1/2\,NS$ and $D_{2.5}W$). Hypertonic solutions have higher osmolality than blood, moving fluid out of cells (examples include $D_5NS$ and $D_5LR$).

Mechanisms of Regulation and Capillary Exchange

Other mechanisms of movement include diffusion, where solutes move from an area of higher concentration to one of lower concentration. Filtration is the movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic pressure. Active transport is a physiologic pump that moves fluid from an area of lower concentration to one of higher concentration, requiring energy in the form of ATP.

Net filtration describes the movement of fluid through capillary walls. Forces favoring filtration include capillary hydrostatic pressure (blood pressure) and interstitial oncotic pressure (water pulling). Forces favoring reabsorption include plasma oncotic pressure (water pulling) and interstitial hydrostatic pressure.

Lifespan Considerations: Gerontologic and Pediatric Differences

In older adults, the percentage of body weight composed of water is decreased. Structural changes in the kidney and decreased renal blood flow lead to a decreased GFR and decreased creatinine clearance. They lose the ability to concentrate urine and conserve water. There is a decrease in renin and aldosterone, an increase in ADH and ANP, loss of subcutaneous tissue, a decrease in the thirst mechanism, and potential musculoskeletal or mental status changes along with incontinence.

Pediatric populations have distinct differences. Premature infants are approximately $90\%$ water, infants are $75-80\%$, and children are $64\%$. Infants have a higher percentage of water in the extracellular fluid and are more vulnerable to disturbances. Kidney concentrating abilities are not mature until age $2$. Their metabolic rate is two to three times higher than adults, and they have a greater body surface area per kilogram, leading to quicker dehydration.

Fluid Volume Pathophysiology

Hypovolemia (Fluid Volume Deficit or FVD) is an osmolality imbalance due to disturbances in body fluid concentration. Sodium levels may be normal, low, or elevated. Isotonic dehydration, where sodium is normal, may result from vomiting, diarrhea, or hemorrhage. Hypertonic dehydration occurs when more sodium than water is lost, causing a hyponatremic volume deficit. Hypotonic dehydration involves less sodium lost than water, causing a hypernatremic volume deficit. Treatment usually involves salt-based crystalloid IV solutions. Dehydration refers strictly to water loss, which always presents with hypernatremia and is treated with free water administration.

Hypervolemia (Fluid Volume Excess or FVE) is an excess of fluid in the ECF. Compensatory mechanisms like the kidneys usually balance water and sodium, but hypervolemia occurs when these fail. Third spacing is a fluid shift from the vascular space to other areas, resulting in edema, decreased cardiac output, and decreased blood pressure. This leads to hypovolemia and can be caused by liver disease, surgery, burns, sepsis, heart failure, or IV fluids. Treatment is often isotonic IV solutions.

Nursing Assessment and Case Study

Assessment data for FVD includes mild loss (orthostatic hypotension, increased heart rate, restlessness), moderate loss (confusion, irritability, extreme thirst, nausea, cool clammy skin, decreased UOP of $10-30\,ml/hr$, and elevated specific gravity), and severe loss (decreased cardiac output, unconsciousness, hypotension, weak or absent pulses). Assessment for FVE includes tachypnea, dyspnea, crackles, bounding pulse, hypertension, distended neck veins, acute weight gain, and pulmonary edema.

In infants, mild dehydration presents as tachycardia and mottled/dry skin. Moderate dehydration shows irritability, lethargy, decreased turgor, and sunken eyes/fontanels. Severe dehydration includes coma, low BP, weak pulse, and deeply sunken fontanels. A Case Study involving five-year-old Emma Ozaki illustrates these points. She presented with a 2-day history of profuse diarrhea, listlessness, poor skin turgor, soft sunken eyes, and dark circles. She had decreased, dark urination, lack of tears, a pulse of $114\,bpm$, respirations of $22/min$, and BP of $98/66\,mmHg$. A fingerstick showed a hematocrit of $58\%$. Bowel sounds were hyperactive, and she was drinking only electrolyte solutions and water.

Clinical Testing and Laboratory Data

A Basic Metabolic Panel (BMP) measures chloride (fluid balance), potassium, sodium, calcium, glucose, creatinine (muscle wear and tear waste), and BUN (blood urea nitrogen). A high BUN to creatinine ratio can indicate dehydration, kidney disease, or CHF. A low ratio can indicate malnutrition or liver disease. Urine specific gravity ranges from $1.005$ to $1.030$.

A 24-hour urine collection checks kidney function. The procedure requires the patient to urinate in the toilet first thing in the morning, then collect all urine for the next 24 hours including the first urination of the following morning. The container must be kept cool, and the patient should avoid stress, strenuous activity, and foods like coffee, tea, bananas, citrus, and vanilla. A fluid challenge may be ordered to differentiate oliguria due to decreased renal blood flow versus decreased renal function. It involves administering $100-200\,ml$ of NS over $15$ minutes; if BP and UOP increase, renal function is normal; if UOP remains low, renal function is decreased.

Nursing Interventions and Fluid Replacement

Interventions for FVD involve recording I&O, monitoring daily weights (where a $2.2\,lb$ gain equals $1\,L$ of fluid), vital signs, and safety precautions. For infants and children, oral rehydration therapy (ORT) is preferred for mild to moderate loss, using $40-50\,ml/kg$ of solutions like Pedialyte, Infalyte, or Ricelyte. IV fluid boluses for children are typically $10$ to $20\,ml/kg$. In hypervolemia, interventions include elevating the head of the bed (HOB), monitoring for dyspnea, administering diuretics, and potential sodium or fluid restrictions.

Blood transfusions may involve packed red blood cells (RBCs) to restore volume, whole blood, or platelets (common for transplants and cancer). Hematocrit is the ratio of RBCs to total blood volume, and Hemoglobin is the iron-containing protein that transports oxygen.

Specific Electrolyte Imbalances

Sodium ($135-145\,mEq/L$) manages muscle contraction and nerve impulses. Hypernatremia causes headache, confusion, increased BP, and hot dry skin; it is managed by gradual hypotonic IV fluids. Hyponatremia leads to anorexia, muscle cramps, twitching, and potential seizures; it is managed by sodium replacement or water restriction.

Potassium ($3.5-5.0\,mEq/L$) manages heart depolarization and skeletal muscle stimulation. Hyperkalemia involves tall peaked T waves, widened QRS, and cardiac arrest; it is treated with calcium gluconate, insulin/glucose, or Kayexalate. Hypokalemia causes dysrhythmias (flat/inverted T waves), confusion, and decreased GI motility; it is managed with potassium replacement if UOP is adequate. High potassium foods include bananas, oranges, spinach, potatoes, and chocolate.

Chloride ($95-105\,mEq/L$) works with sodium to maintain electrical neutrality and forms hydrochloric acid. Hypochloremia presents as agitation and muscle spasms; hyperchloremia presents as tachypnea, lethargy, and Kussmaul respirations. Chloride is found in tomatoes, olives, and leafy vegetables.

Calcium, Magnesium, and Phosphorus

Calcium ($9-11\,mg/dL$) is used for blood clotting and bone formation. Hypercalcemia causing bone pain, kidney stones, and polyuria is treated with hydration and parathyroidectomy. Hypocalcemia causes tetany, Trousseau's sign (hand contraction with BP cuff), and Chvostek's sign (facial twitching); it is treated with calcium gluconate. Vegan sources include kale, broccoli, and tofu.

Magnesium ($1.5-3.0\,mg/dL$) is used for protein synthesis and neuromuscular function. Hypomagnesemia causes hyperactive DTRs and Trousseau/Chvostek signs. Hypermagnesemia causes lethargy and depressed respirations; it is treated with calcium gluconate. Foods include cashews, avocados, and oatmeal.

Phosphorus (normal $< 2.0\,mg/dL$) has an inverse relationship with calcium. Hypophosphatemia is associated with alcoholism and hypercalcemia. Hyperphosphatemia is rare, usually seen in renal disease. High phosphorus foods include dairy, processed meats, and cola.

Acid-Base Balance and ABGs

Normal plasma $pH$ is $7.35-7.45$. Acidosis is $< 7.35$ and Alkalosis is $> 7.45$. Buffers include chemical buffers (seconds), respiratory system (minutes via $CO_2$ regulation), and renal system (hours/days via bicarbonate or $HCO_3^-$). In metabolic acidosis, the body excretes $H^+$ and conserves $HCO_3^-$. In respiratory acidosis, the body increases respiratory rate to "blow off" $CO_2$.

Standard ABG values are $pH\,7.35-7.45$, $P_aCO_2\,35-45\,mm\,Hg$, and $HCO_3^-\,22-26\,mEq/L$. The ROME mnemonic defines interpretation: Respiratory Opposite (if $pH$ is up and $PCO_2$ is down, it is alkalosis) and Metabolic Equal (if both are up, it is alkalosis). Respiratory acidosis is always a respiratory problem with $CO_2$ retention (e.g., drug overdose, obstruction). Respiratory alkalosis is always due to hyperventilation. Metabolic acidosis involves the kidneys and can cause hyperkalemia as $K^+$ moves from ICF to ECF.

Practice Problems

Problem 1: $pH\,7.31$, $PCO_2\,55\,mm\,Hg$, $HCO_3^-\,35\,mEq/L$. Problem 2: $pH\,7.31$, $PCO_2\,35\,mm\,Hg$, $HCO_3^-\,20\,mEq/L$. Problem 3: $pH\,7.31$, $PCO_2\,25\,mm\,Hg$, $HCO_3^-\,20\,mEq/L$. Problem 4: $pH\,7.48$, $PCO_2\,25\,mm\,Hg$, $HCO_3^-\,20\,mEq/L$. Problem 5: $pH\,7.48$, $PCO_2\,45\,mm\,Hg$, $HCO_3^-\,33\,mEq/L$. Problem 6: $pH\,7.48$, $PCO_2\,55\,mm\,Hg$, $HCO_3^-\,33\,mEq/L$.